Reinforcement cages such as pile cages are used in a wide range of civil engineering and construction applications, principally in the formation of concrete piles in the construction of buildings, underground car parks, road or rail or other bridges, and other structures. Pile cages not only provide reinforcement for the concrete of the pile, but they also provide a means of attaching or anchoring part(s) of the building, bridge or other structure to the built pile itself.
As used herein, the term “pile cage” means a generally cylindrical, or alternatively other cross-sectional shaped, assembly or network of a plurality (typically at least about 4, 5 or 6, or possibly more than six) of (usually) metallic, e.g. steel, cage bars extending in a generally longitudinal direction (defined as a direction parallel to the axis of the pile to be formed around the cage) and anchored together or interconnected by one or more frame elements, e.g. one or more wires or other supporting frame members, which maintain the relative positioning, separation and alignment of the cage bars. Thus, a pile cage is a relatively stiff, structurally relatively stable assembly, and is often manufactured off-site in a dedicated assembly plant and transported by vehicle to the building site ready for use in the building of the required piles.
Typically a hole of the required size and cross-sectional shape to form the pile is formed in the ground by drilling and is then at least partially lined (to prevent wall collapse) with a reusable casing. A pile cage is then lowered into the lined hole, and wet concrete is then poured therein, embedding the cage within it. The casing is then withdrawn, for re-use in the building of another pile, while the concrete is still wet, and the concrete is then allowed to cure to form the pile. Frequently, however, it is necessary to form particularly tall piles, i.e. of a height greater than the length of a single individual pile cage. In this case it is common practice to splice together at least two pile cages end-to-end, i.e. to connect the top end of a lower pile cage to the bottom end of an upper pile cage. Frequently as many as three, four or even more individual pile cage sections are spliced together end-to-end in a corresponding manner to form a single unified pile cage of the required total length. The complete pile cage assembly is typically built up incrementally as the individual cage sections are spliced together and lowered in a stagewise manner into the lined hole. During the splicing and stagewise lowering operation each successive pile cage section is generally accurately positioned (e.g. using a crane) directly above an exposed upper portion of the pile cage section below it, then spliced thereto by whatever means is being employed for that job.
In the formation of concrete structures other than circular piles, reinforcement cages of other types may be used. For example, diaphragm walls, such as those of rectangular, or even L-shaped or T-shaped, cross-section may be formed in an analogous manner to cylindrical piles, but instead of using a pile cage as such, a reinforcement cage of an appropriate alternative shape and configuration is used. Such an alternative form of reinforcement cage used to form diaphragm walls may thus be termed a “diaphragm wall cage”.
Splicing together pairs of reinforcement cages, whether of the pile, diaphragm wall or other type, is however not a simple matter, and the job comes with ever increasing health and safety risks that have to addressed. Various methods and devices for splicing together reinforcement cages are therefore known, and in recent years these have not only been aimed at simplifying the mechanical job of uniting adjacent reinforcement cage lengths, but also to do so with greater attention paid to health and safety risks, such as of the need to avoid workers having to place their hands or arms inside the interior space within a part-assembled reinforcement cage during a splicing operation.
One such well-known and currently commercially widely used system for splicing together adjacent pile cages is disclosed in published International Patent Application WO2007/068898 (also published as EP1963579A). Here a supported lower pile cage is fitted with a circumferential suspension band, e.g. by welding to the longitudinal cage bars, and the upper pile cage is fitted with at least one support plate (preferably a plurality, e.g. three, thereof, equi-angularly spaced) such as by welding thereof to a respective cage bar. Each support plate has a screw-threaded aperture therein, into which is screwable from outside the cage a respective suspension bolt. Once the upper pile cage has been lowered (e.g. by a crane) and accurately positioned above the lower pile cage with the support plates on the upper pile cage positioned adjacent the suspension band on the lower pile cage and the axis of the threaded apertures in the support plates located below the suspension band, the suspension bolts are inserted into their respective threaded apertures in the support plates so as to protrude radially inwardly of the pile cages (i.e. transverse to the longitudinal axes of the pile cages and directed generally towards those axes) and beneath the suspension band on the lower pile cage. Once screwed home, the suspension bolts thus collectively abut the underside of the suspension band and so serve to carry the lower pile cage beneath the upper pile cage as the latter is lifted or craned into a new position, such as a new location on site or to be lowered into a casing ready for pouring of concrete to form a pile around the combined pile cages.
In an alternative configuration to the above, the support plates may instead be provided on the lower pile cage and the suspension band on the upper pile cage. In this case, in the step of lowering and positioning the upper pile cage above the lower pile cage, the axis of the threaded apertures in the support plates is located above the suspension band, so that once the suspension bolts have been inserted into their respective apertures in the support plates and screwed home, the suspension bolts thus collectively abut the topside of the suspension band. In this manner the suspension band (on the upper pile cage) still serves to carry the lower pile cage beneath the upper pile cage as the latter is lifted or craned into a new position, it simply being that the support plates and the suspension band have been inverted in their relative positioning on the respective upper and lower pile cages.
We have found that in practice this known pile cage splicing system has several disadvantages:
These known radially-inwardly extending suspension bolts are anchored and supported substantially only at their radially outer ends, i.e. in their respective support plates only. This “encastré” cantilevering means that in the event that load is placed on a bolt at a point a distance “x” away from its cantilevered anchoring in its respective plate, then any lateral deflection suffered by the bolt at that loading point is proportional to “x3”. Thus, any loads applied to the bolts at increasing radial distances from their respective anchoring points in the respective support plates can give rise to especially large lateral bolt deflections. This can be critical for suspension bolts of a given diameter and/or strength, since even modest loadings on such bolts at increasing distances from their respective support plates can cause moderate or even excessive bending of the bolts, or even their breaking altogether. Such mechanical failure of at least some of the suspension bolts means that they can no longer be expected to properly support and carry the suspension band of the lower pile cage, which as a result may all too easily slide off at least some of the suspension bolts or even the entire collective support provided by the complete array of bolts. At worst the lower pile cage may even fall off it completely, the suspension band having slid off the bolts entirely, and become separated from the pile cage assembly. Clearly this can lead to highly risky working conditions for site workers and may have highly serious consequences for health and safety.
This application of loading forces, especially eccentric loading forces, on the suspension bolts at increasing distances from their respective anchoring locations on the support plates may be commonly encountered in any instance where a given pile cage is free to move laterally (i.e. transversely relative to the longitudinal direction of the pile cage) with respect to an adjacent pile cage. Such freedom of movement may occur for example where a pile cage has been damaged, e.g. bent, in transit or in storage, possibly as a result of mis-handling or lack of supervision. In the case of pile cages which incorporate spacers that are used to centralise adjacent pile cages with respect to one another and/or within a casing, it may also result from damage or flattening to such spacers. It may also result from asymmetrical misalignments in the relative configurations of the cage bars of adjacent pile cages where one is “cranked” with respect to the other, i.e. the cage bars of one cage in an end region thereof are configured so as to be bent to lie a short distance radially inwardly of the main body of the other cage, in order to improve the flow of liquid concrete into the cage when poured therein and also to assist in the alignment of one pile cage with respect to the next.
Moreover, the exertion of an excessive bending load on one suspension bolt only can easily lead to overloading of other bolts at other circumferential locations around the cage, possibly leading to progressive failure of all the bolts. It is thus a potentially particularly serious shortcoming of this known system of splicing pile cages that relies on cantilevered suspension bolts to perform a stable and reliable cage suspending function.
Corresponding problems can occur in the use of known reinforcement cages of other types, including diaphragm wall cages, which are constructed and utilised in an analogous manner and using corresponding principles to pile cages.
It might be suggested that an amelioration of these problems might be to use longer and/or thicker or stronger suspension bolts. However in practice this is not a good solution. For one thing, it would require the use of heavier and bulkier components and equipment, which not only increases cost, but also makes manual fixing and screwing home of the suspension bolts more difficult and time consuming, which may be especially troublesome in the case of congested cages where small circumferential gaps between bars may not allow the insertion of thicker bolts. For another thing, it does not address the fundamental problems arising from overloading and excessive bending of even such longer and/or stronger bolts as a result of loading points increasingly spaced from their “encastré” cantilevered fixings in the respective support plates, which can still occur for the practical reasons discussed above. Furthermore, the use of longer bolts would generally be undesirable anyway, since they would hinder the placement into the interior of the spliced cages, once in position in the relevant hole in the ground, of the (circular) concreting tube (“tremmie”) used to fill the hole with wet concrete during the pouring stage of the pile- or wall-forming operation.
Another practical problem with known cage splicing devices such as those of EP1963579A above is that the use of threaded bolts inserted in the threaded apertures of the support plates requires very precisely engineered components made from high-quality, high carbon-footprint materials, which increases manufacturing costs. It can sometimes occur that as-manufactured threaded components may not always fit together exactly, and moreover during transport or while on-site threads can become damaged, e.g. by impact with other components, or clogged with dirt or debris, all of which issues can cause unnecessary delays in the running of an efficient pile building operation.
A further practical problem associated with the use of suspension bolts as in EP1963579A is that the use of bolts as separate components means that any given cage splicing job relies on the provision to site and utilisation of loose items which can sometimes get dropped or lost, even down the hole above which the cages are being spliced. Again, this can result in delays and also unnecessary wastage of usable components.